Processing of telephone signals typically requires inclusion in the telephone signals of standard signaling tones such as DTMF (dual tone multi-frequency) tones or call progress tones. However, when telephone signals are transmitted according to certain techniques, such as VOIP (voice over internet protocol), it may be desirable to suppress signaling tones within the voice band and to replace those tones with out of band signaling, or with standardized special signaling data frames, as proposed in RFC (Request for Comments) 2833, published by the Internet Engineering Task Force (IETF).
Suppression or “clamping” of frames that include a signaling tone may entail trade-offs. A tone having a duration of less than 30 to 40 milliseconds (e.g., 3 to 4 10-millisecond data frames) may not be considered to be a valid tone, and a typical clamping regime may not call for clamping such relatively short tones. A longer tone (30 to 40 milliseconds or more) may typically be clamped, at least in part. Since delay in frame transmission may be undesirable in real-time communication sessions, it has been proposed to transmit the first three frames of a tone signal, and then to clamp the fourth and succeeding frames. A possible drawback to this proposal may be that the tone duration in the first three transmitted frames may be sufficiently long to trigger tone detection at a destination for the signal, which may interfere with proper reception of the signal.